As an electrical inspector, you must know that Splices of bare copper cables used underground in ground grids must have very low resistance and be very reliable. Periodic inspection of these splices is impossible after installation. For these reasons, Consultants often specifies that exothermic welds be used in these splices.

A- Definition

An exothermic weld is a fusion weld that takes place in an exothermic weld mold (or graphite mold), as shown in Figure 1.

Figure 1. Exothermic weld mold

B- Execution Method

As in the above definition, An exothermic weld is a fusion weld that takes place in an exothermic weld mold (or graphite mold), this will be done by following the below steps (see fig 2 &3):

The conductors are placed in the weld cavity.

The weld metal powder consists of powdered copper oxide and aluminum. This mixture is placed in the mold, supported by a steel disk.

A starting powder is placed on top of the weld metal powder. When ignited, the starting powder produces a temperature that is higher than the ignition point of the weld metal powder (815°C).

When the weld metal powder ignites, the resulting thermite reaction produces a molten copper and aluminum oxide mixture called fill that melts the steel disk and flows through a tap hole into the weld cavity. The super-heated fill flows over the conductors, causing them to melt and form a fusion weld.

After the weld is completed, the mold is separated from the new connection. Figure 4 shows a cross section of an exothermic T-weld.

Different types of molds are used to make different kinds of welds. A figure 5 illustrates various types of exothermic welds.

Figure 5. Different types of exothermic welds

E- Inspection

A good exothermic weld is usually gold or bronze in color.

A wire brush may be used to expose the color of the splice.

The splice surface should be quite smooth with no deposits of slag (Figure 6A).

A few holes the size of a pin can be present but a large number of holes are unacceptable (Figure 6B).

The conductor ends should be completely covered by fill (Figure 6C).

Using old, worn out molds can cause leakage of the fill (Figure 6D). Excessive leakage is also unacceptable.

All the welds shown on Figure 6 are unacceptable and should be rejected.

Figure 6. Unacceptable exothermic welds

7.2 Compression Tools and Dies

Most Electrical Contractors use mechanical and hydraulic tools to install compression connectors. Large mechanical and hydraulic installation tools sometimes use interchangeable dies, giving the tool the ability to install a wide range of connector sizes and types.

After finishing splicing process, the splices must be insulated by one of the below two methods:

1- Plastic Tape

Plastic tape is commonly used to insulate bare or stripped copper conductors after they have been spliced together. After insulating, no part of the bare copper conductor should be visible. For ground conductors, this tape should be colored green.

2- Heat-shrinkable Sleeve

Heat-shrinkable sleeves are also used to insulate bare copper conductors. The sleeve is placed over the splice and then heated. Heat makes the sleeve shrink to form a tight, waterproof seal around the spliced area. This material is marked to indicate the size conductor for which it is used.

In the next article, I will explain how to identify Grounding Connectors And Bushings?. Please, keep following.